1. Field of the Invention
The present invention relates to method and apparatus for processing a substrate and a plate used therein, and more particularly to method and apparatus for processing a substrate that enable surface roughening on a silicon substrate used in a solar cell or the like, and a plate used therein.
2. Description of the Related Art
A solar cell converts incident light energy into electrical energy. The solar cell is classified into crystal-based, amorphous-based, and compound-based solar cells depending on materials used. Among others, crystalline silicon solar cells account for a large percentage of the solar cells currently available on the market. The crystalline silicon solar cell is further classified into a single-crystalline silicon type and a multi-crystalline silicon type. A silicon solar cell of a single-crystal type has an advantage that efficiency can be readily improved due to the substrate of high quality, but has a drawback that the manufacturing cost of the substrate is high. On the contrary, a multi-crystalline silicon solar cell has a drawback that efficiency cannot be readily improved due to the substrate of inferior quality, but has an advantage that the manufacturing cost is low. In addition, improvement in quality of the multi-crystalline silicon substrate and advancement in the cell fabrication technique in recent years made it possible to achieve conversion efficiency of approximately 18% in the research level.
On the other hand, while multi-crystalline silicon solar cells have been available on the market because they can be manufactured at a low cost through mass-production, the demand is now on the increase due to growing concern about environmental issues in recent years, and there is a need to achieve higher conversion efficiency at a low cost.
There have been made many attempts regarding solar cells to improve conversion efficiency to electrical energy. One of such attempts relates to a technique to reduce reflection of light incident on the substrate, by which conversion efficiency to electrical energy can be improved by reducing reflection of light on the surface.
In a case where a solar cell is fabricated from a silicon substrate, reflection can be reduced to some extent by etching the surface of the substrate with alkaline aqueous solution, such as sodium hydroxide, to form fine textures (concavities and convexities) on the surface of the substrate. In a case where a single-crystalline silicon substrate having a (100)-plane orientation, a group of myriads of pyramids called a texture structure can be formed on the surface of the substrate by the method described above.
Etching with the use of alkaline aqueous solution, however, depends on the plane orientation of crystals, and for this reason, in a case where a solar cell is fabricated from a poly-crystalline silicon substrate, a group of pyramids cannot be formed homogeneously, which raises a problem that overall reflectance cannot be reduced effectively. When the textures cannot be formed homogeneously, incident light cannot be taken into the solar cell effectively, and the solar cell has little hope of improving photoelectrical conversion efficiency.
In order to eliminate such a problem, there has been proposed an idea of texturing the surface by forming fine textures through the reactive ion etching method when a solar cell element is fabricated from multi-crystalline silicon (see Japanese Laid-Open Patent Application No. 102625/1997, etc.). In other words, this idea is to reduce reflectance of a solar cell using multi-crystalline silicon more effectively by forming fine texture homogeneously on multi-crystalline silicon regardless of anomalous plane orientations of the crystals.
A substrate processing apparatus used in the reactive ion etching method is generally of a parallel plate counter-electrode type, wherein an RF voltage plate is provided on the side where the substrate is placed and the electrode on the other side and the internal sidewall are connected to ground. The interior of the chamber is evacuated, then the substrate is added by the RF voltage and is subjected to plasma etching while a constant pressure is maintained by introducing an etching gas, and a pressure in the interior of the chamber is restored to atmospheric pressure after the etching is completed.
Because of the procedure described above, waiting times for evacuation and leaking to atmospheric pressure are long in a reactive ion etching apparatus. Moreover, the area of the solar cell itself is large. Hence, there is a problem that the manufacturing cost of a solar cell is increased because only a small number of substrates can be processed at a time.
Accordingly, in the case of using a reactive ion etching apparatus in the fabrication sequence of a solar cell, a way in which the number of substrates to be processed at a time is increased at high tact while ensuring homogeneity of textures formed the surface of the substrate, or a way in which the area of a substrate to be processed is increased is crucial.
It is therefore an object of the invention to provide the method and apparatus for processing a substrate that make it possible to form textures homogeneously on the surface of a semiconductor substrate, in particular, a substrate used in a solar cell, at high throughput, and a plate used therein.